A typical digital mobile phone network is shown in FIG. 1 in which, broadly speaking, infrastructure elements above dashed line 10 are found in 2G phone networks and the elements below line 10 are the additional elements incorporated in 2.5G and 3G networks to handle packet data transmission. The basic structure of FIG. 1 is common to all digital mobile phone networks, but for convenience the network is labelled using mainly GSM terminology.
In FIG. 1 a radio mast 12 is coupled to a base station 14 which in turn is controlled by a base station controller 16. A mobile communications device 18 is shown in two-way communication with base station 14 across a radio or air interface 20, known as a Um interface in GSM and GPRS (General Packet Radio Service) networks and a Uu interface in cdma2000 and WCDMA networks. Typically at any one time a plurality of mobile devices 18 are attached to a given base station, which includes a plurality of radio transceivers to serve these devices.
Base station controller 16 is coupled, together with a plurality of other base station controllers (not shown) to a mobile switching centre (MSC) 22. A plurality of such MSCs are in turn coupled to a gateway MSC (GMSC) 24 which connects the mobile phone network to the public switched telephone network (PSTN) 26. A home location register (HLR) 28 and a visitor location register (VLR) 30 manage call routing and roaming and other systems (not shown) manage authentication, billing. An operation and maintenance centre (OMC) 29 collects the statistics from network infrastructure elements such as base stations and switches to provide network operators with a high level view of the network's performance. The OMC can be used, for example, to determine how much of the available capacity of the network or parts of the network is being used at different times of day.
The above described network infrastructure essentially manages circuit switched voice connections between a mobile communications device 18 and other mobile devices and/or PSTN 26. So-called 2.5G networks such as GPRS, and 3G networks, add packet data services to the circuit switched voice services. In broad terms a packet control unit (PCU) 32 is added to the base station controller 16 and this is connected to a packet data network such as Internet 38 by means of a hierarchical series of switches. In a GSM-based network these comprise a serving GPRS support node (SGSN) 34 and a gateway GPRS support node (GGSM) 36. It will be appreciated that both in the system of FIG. 1 and in the system described later the functionalities of elements within the network may reside on a single physical node or on separate physical nodes of the system.
In a 2.5G or 3G network mobile device 18 may provide more than a simple voice connection to another phone. For example mobile device 18 may additionally or alternatively provide access to video and/or multimedia data services, web browsing, email and other data services. Logically mobile device 18 may be considered to comprise a mobile terminal (incorporating a subscriber identity module (SIM) card) with a serial connection to terminal equipment such as a data processor or personal computer. Generally once the mobile device has attached to the network it is “always on” and user data can be transferred transparently between the device and an external data network, for example by means of standard AT commands at the mobile terminal-terminal equipment interface. Where a conventional mobile phone is employed for mobile device 18 a terminal adapter such as a GSM data card may be needed.
Third generation technology encompasses Wide-band Code Division Multiple Access (WCDMA) in Europe and Japan, cdma2000 in the USA, and Time Division Duplex CDMA in China. In a 3D network the base station controller 16 is known as a Radio Network Controller (RNC) and base station 14 is known as a Node B; the network as a whole is known as a UMTS (Universal Mobile Telecommunications System) network. Such networks are the subject of standards produced by the Third Generation Partnership Project (3GPP) and technical specifications for such networks, including TS 23.060 and TS 23.107 which are hereby incorporated by reference, can be found on the 3GPP website.
In a 3G system radio interface resources are shared dynamically between speech and data as a function of service load and operator preference. In addition the radio interface also carries signalling including control messages for managing the network and its resources. The demands on the network depend upon the demand for traffic made by an individual mobile station and the load imposed on the network by other traffic in the same and nearby cells. The packet data transmission has an associated quality service (QoS) profile which is negotiated with the network in accordance with the available radio resources. The network attempts to provide adequate resources to support the negotiated quality of service and the data transmission radio priority is determined based upon this.
In a 3G UMTS network (see 3G TS 23.107) four different QoS classes (or traffic classes) are proposed, conversational class, streaming class, interactive class and background class. The most significant distinction between these classes is the sensitivity of the traffic to any delays in the network. Thus the conversational class and streaming class data services provide a guaranteed bit rate and a guaranteed transfer delay and preserve the time relation between information entities of the stream or conversational pattern. The guaranteed bit rate guarantees delivery of a number of bits within a period of time. The guaranteed (maximum) transfer delay limits the time between requesting transfer of a service data unit or packet and its delivery, and is generally specified for one or more fixed service data unit sizes.
In a packet switched network different packets may take different routes to their destination and, depending upon other traffic within the network, may be delayed to a greater or lesser extent or even entirely lost. Circuit switched voice connections are also given priority over data connections at the radio interface and thus the number of voice calls can also affect the data traffic. The characteristics of packet switched data networks mean substantial spare capacity is needed to ensure that data packets carrying a payload of streamed media data, such as audio or video data, arrive in the right order and sufficiently regularly to provide smooth playback.
It can therefore be appreciated that the transmission of streamed media data in packets over 2.5G and 3G mobile phone networks presents a significant challenge, particularly when the network is operating close to capacity, as it frequently will be. Notwithstanding this public expectation for these networks are for the delivery of smooth, high-quality multimedia services.
U.S. Pat. No. 6,029,064 describes a mobile audio programme selection system in which a mobile phone user dials up a service when information is required and listens to an announcement in real time. However such a service is costly for the network operator to provide and for the user as such information is most likely to be required during peak traffic times. Furthermore the system is limited to information in an audio format.
There is therefore a need for improved methods and apparatus for delivering such services whilst maintaining adequate quality, particularly when the phone network is heavily loaded.